In electric resistance welded steel pipes manufactured such that first a steel strip is continuously formed and then subjected to electric resistance welding, large bending strain is introduced during the pipe manufacturing, causing a significant increase in yield strength YS after the pipe manufacturing. Therefore, a precipitation-hardened steel sheet having such a tendency that the yield strength YS is significantly increased by applied strain has been conventionally incapable of being used as a material for low-yield ratio high-strength electric resistance welded steel pipes without heat treatment subsequent to pipe manufacturing. Therefore, a solid solution-hardened steel sheet with a composition system having an increased C content and an increased Mn content has been conventionally used as a material for low-yield ratio high-strength electric resistance welded steel pipes.
For example, Japanese Unexamined Patent Application Publication No. 2004-217992 describes a method of manufacturing an electric resistance welded steel pipe such that a base steel pipe having a composition containing 0.0002% to 0.5% C, 0.003% to 3.0% Si, 0.003% to 3.0% Mn, 0.002% to 2.0% Al, 0.003% to 0.15% P, 0.03% or less S, and 0.01% or less N is heated to a temperature not lower than the Ae3 point and not higher than 1,300° C., subjected to reducing rolling at a finishing rolling temperature of (the Ae3 point—50° C.) or higher, subjected to cooling within 2 seconds thereafter, cooled to (the Ae3 point—70° C.) at a rate of 5° C./sec. to 20° C./sec., and cooled to (the Ae3 point—150° C.) at a rate of 1.0° C./sec. to 20° C./sec. This allows a steel pipe which has a pipe body having a fine, uniform grain size, with only the surface layer further refined, and an excellent strength-ductility balance to be obtained.
However, in Japanese Unexamined Patent Application Publication No. 2004-217992, heating a pipe body and subjecting the pipe body to reducing rolling are essential requirements. Since the pipe body is heated, there are problems that surface quality is low and production cost is significantly high.
Furthermore, to ensure high strength for a solid solution-hardened steel sheet, the amounts of C and Mn need to be large. Therefore, there are problems that quality such as strength varies significantly due to unavoidable fluctuation in hot rolling conditions, the inner wrap of a coil tends to be softer than the outer wrap after coiling, and quality such as strength varies significantly depending on coil locations. In particular, in heavy-gauge steel sheets (thick products) with a thickness of 10 mm or more, this tendency is remarkable. Therefore, it is difficult to stably manufacture a high-strength electric resistance welded steel pipe having a small variation in quality from a solid solution-hardened steel sheet without heating during pipe manufacturing, and that is particularly difficult in heavy gauges. In addition, heating a pipe during pipe manufacturing requires a heating facility and causes a reduction in productivity. Furthermore, there is a problem in that since scale is formed by the heating, many surface defects are caused by the contact of steel with a roll to deteriorate the surface condition. Therefore, a method capable of manufacturing a low-yield ratio high-strength electric resistance welded steel pipe without heating has been strongly desired.